They would at that time be subject to repeated attacks by insect-eaters, and, even if finally rejected, would often receive a fatal injury.

Hence arose the necessity for some distinguishing mark, by which the devourers of butterflies in general might learn that these particular butterflies were uneatable; and every variation leading to such distinction, whether by form, colour, or mode of flight, was preserved and accumulated by natural selection, till the ancestral Heliconoids became well distinguished from eatable butterflies, and thenceforth comparatively free from persecution.

Then they had a good time of it.

They acquired lazy habits, and flew about slowly.

They increased abundantly and spread all over the country, their larvae feeding on many plants and acquiring different habits; while the butterflies themselves varied greatly, and colour being useful rather than injurious to them, gradually diverged into the many coloured and beautifully varied forms we now behold.
But, during the early stages of this process, some of the Pieridae, inhabiting the same district, happened to be sufficiently like some of the Heliconidae to be occasionally mistaken for them.